1. Field of Invention
My invention relates to the field of radio frequency analysis, and more specifically to the area of microwave and millimeter-wave spectrum analysis.
2. Brief Description of the Prior Art
The prior art shows a wide variety of methods for assessing the spectral content of radio frequency signals. The earliest methods employed tunable resonant devices. The output of these devices was monitored as the resonance frequency was adjusted. A peak or null in the output identified the input frequency. Such as scheme could only operate when a single input frequency was present and required manual (and slow) scanning of the band of interest.
The most common method of spectrum analysis employs a superheterodyne receiver which is swept over the frequency band of interest. This method accommodates a multitude of frequencies in the signal, allowing each to be recognized independently. Unfortunately, the frequencies are studied sequentially, so that pulsed signals are easily missed, as such signals may be off when the receiver visits their frequencies.
Other approaches, often identified as IFM (instantaneous frequency measurement) devices, measure relative phase between the signal and a delayed version of the signal. Such systems cover all frequencies simultaneously, but can only tolerate one frequency in the signal at any given time. Various improvements made to this basic approach have allowed only modest input diversity.
An exhaustive approach which overcomes the objections cited above is to construct a bank of parallel filters, said filters being driven by the signal simultaneously. The radio frequency power in each filter output is monitored to obtain the desired spectrum. Such a scheme is impractical due to the complexity, bulk, and cost of a large number of filters.
Snapshot transform methods are also used. Here, a record of the recent signal history is first obtained. The samples are then transformed via the Fourier transform. While the transform may be done at low frequencies, the sampling for such a system must be done at the signal frequencies. As such, input frequencies must be below the microwave band.
Acoustooptic or acoustic methods, which exploit wave propagation to implement a Fourier transform, also allow multiple frequencies and cover the entire band at once; however, they too are constrained in bandwidth by their input transducers. Practical devices are limited to a few gigahertz.
Still another microwave and millimeter-wave method, used by the Fourier spectrometer, mechanically scans out the autocorrelation function of the input signal. Said autocorrelation function is Fourier transformed to produce the power spectrum. The Fourier spectrometer consists of a radio frequency Michelson interferometer in which one reflector is moveable, and a detector which is placed at the output of said interferometer. For any position of the moveable reflector, the detector responds to the signal plus a delayed replica of the signal. The detector's slow square-law response causes the detector output to be a measure of the signal's autocorrelation at the delay between the two signals. By moving the reflector, the delay is varied and the autocorrelation is thereby scanned out. The Fourier spectrometer usually uses freespace propagation, rather than guided propagation. The Fourier spectrometer approach suffers from two major drawbacks: First, it is mechanically scanned, making it slow and prone to mechanical failure. Second, the scanning introduces an ambiguity which causes signal modulation to manifest itself as spurious frequencies in the output.